The present invention relates generally to electronic circuitry and more particularly to isolation circuitry such as used in telephone line interface equipment.
The U.S. Federal Communications Commission and counterpart regulatory agencies in other countries require that user telephone equipment be isolated from the telephone network. The primary purpose of this requirement is to protect the network from damage due to such occurrences as faulty user devices and inadvertent shorts between user equipment and power lines. Specific requirements for such isolation vary somewhat from country to country (for example the U.S. requires 1500-volt isolation while other countries require as much as 4000-volt isolation), but the requirements are invariably stringent.
A typical telephone interface circuit (also known as a direct access arrangement or DAA) couples to the two-line network, and includes a duplexing circuit, referred to as a hybrid, that provides separate transmit and receive channels. The hybrid, which typically comprises a multiple winding transformer, may be balanced or unbalanced. The hybrid for telephones is typically unbalanced so as to provide a sidetone that allows the speaker to hear his/her own voice in the receiver. The hybrid for a modem or the like is balanced so that none of the transmitted signal is communicated into the receive channel. The DAA also provides a ring detection signal to the user equipment, and includes an off-hook relay for maintaining an open circuit on the network until the user device goes off-hook.
The vast majority of prior art DAAs use isolation transformers, although there has been some recent recognition that at least certain paths can be isolated using other techniques. For example, U.S. Pat. No. 4,864,605 to Ramsey et al. shows the use of a coupling transformer in the main signal path and opto-isolators in the ring detect, line condition detect, and off-hook detect circuitry. Somewhat analogously, U.S. Pat. No. 4,417,099 to Pierce discloses the use of opto-isolators in the digital portion of the data paths of a line-powered modem.
Transformers, while effective for isolation and coupling, are expensive, and are bulky and heavy relative to most solid state devices. Moreover, isolation transformers tend to be a leading source of failure in equipment which is subject to relatively heavy usage. In such applications, a transformer is most likely damaged internally or to be separated from its mounting and electrical connections by mechanical shock or vibration, due primarily to its mass.
One approach to eliminating the isolation and hybrid transformers is embodied in a DAA marketed by Cermetek Microelectronics, Inc. under the part number CH1817. Isolation is provided by two sets of high-value isolation resistors and associated components which act to isolate the send and receive portions of the user equipment from the telephone network.